1. Field of the Invention
The present invention relates to a thermal transfer-receiving sheet used together with a thermal transfer recording ribbon having a thermal transfer dye layer. In particular, it relates to a thermal transfer-receiving sheet that has good detachability from a thermal transfer recording ribbon during printing and high color density and lightfastness. It also relates to a high-grade thermal transfer-receiving sheet that suppresses image bleeding even after being stored in a high-temperature, high-humidity environment for a long time, and to a method for making a thermal transfer-receiving sheet.
2. Description of the Related Art
Thermal transfer techniques have been available as the technique of forming color or monochrome images. The thermal transfer techniques use a thermal transfer recording ribbon, which contains a heat-diffusible dye that can be transferred by being melted or sublimated with heat from a thermal head or the like, and a thermal transfer-receiving sheet that has a dye-receiving layer for receiving the dye from the thermal transfer recording ribbon. Thermal transfer techniques are techniques for forming images by arranging a thermal transfer recording ribbon to face the dye-receiving layer of the thermal transfer-receiving sheet and pressing a thermal head against a rear side of the thermal transfer recording ribbon to apply heat so that the dye in form of an image is transferred onto the dye-receiving layer. Such thermal transfer techniques have earned a reputation as a technique for forming images from digital data and achieving gradation comparable to that of silver halide photographs without using any processing liquid such as a developing liquid.
The dye-receiving layer of the thermal transfer-receiving sheet is frequently formed by using as a main component various thermoplastic resins such as polyester, polyamide, acryl, acetate, polyvinyl chloride.vinyl acetate, styrene, polyvinyl butyral, cellulose, and polycarbonate. In particular, Japanese Unexamined Patent Application Publication No. 57-107885 proposes to provide on a substrate a dye-receiving layer that functions as a coating layer containing a saturated polyester, polyvinyl pyrrolidone, and a pigment. Japanese Unexamined Patent Application Publication No. 60-122192 proposes formation of a layer including at least one selected from a polyester resin, a polyamide resin, an acrylic resin, and an acetate resin and at least one selected from a hydrocarbon resin, a fluorine resin, and a silicon resin. Japanese Unexamined Patent Application Publication No. 61-283595 proposes formation of a layer composed of a mixture of a saturated polyester resin and a vinyl chloride-vinyl acetate copolymer. Japanese Unexamined Patent Application Publication No. 62-189195 proposes a receiving layer composed of a styrene resin.
While the object of these proposals has been to improve the color density and lightfastness, all faced a difficulty in obtaining satisfactory results. In particular, discoloration and bleeding of images caused by light, heat, and humidity have frequently occurred and high-definition recorded images could not be maintained for a long period of time. The cause therefor is the recent increase in printing speed of thermal transfer printers. In other words, when proposed thermal transfer-receiving sheets having dye-receiving layers are used in high-speed thermal transfer printers, the dye is not sufficiently dispersed and affixed in the dye-receiving layers due to short printing time. Accordingly, the dye is concentrated in the surface area of the dye-receiving layers of the thermal transfer-receiving sheets, thereby degrading the color density and lightfastness. Moreover, the dye concentrated in the surface area of the dye-receiving layer diffuses with lapse of time by being affected by temperature and humidity and thus severe image bleeding has occurred.
According to a high-speed thermal transfer printer, a thermal transfer recording ribbon and a thermal transfer-receiving sheet may be detached from each other before both are sufficiently cooled. Thus, the thermal transfer-receiving sheet preferably displays good detachability. Proposals for improving the detachability during printing include, for example, Japanese Unexamined Patent Application Publication Nos. 10-309874, 10-337967, 10-337969, and 06-24152.
Japanese Unexamined Patent Application Publication No. 10-309874 proposes a resin composition that contains polymer fine particles having a core/shell structure or a micro domain structure constituted by a polymer of a urethane resin and a polymerizable unsaturated compound, and a receiving layer composed of the resin composition. Japanese Unexamined Patent Application Publication No. 10-337967 proposes a resin composition which is a polymer having a core/shell structure in which the glass transition temperatures of the core and the shell are set within particular ranges, and a receiving layer composed of this resin composition. Japanese Unexamined Patent Application Publication No. 10-337969 proposes a resin composition containing a non-yellowing urethane resin, a resin composition having a core/shell or domain structure in which particles of a vinyl polymer are dispersed in a urethane resin, and a receiving layer a receiving layer composed of this resin composition. Japanese Unexamined Patent Application Publication No. 06-24152 proposes a composite polymeric aqueous dispersion having a core/shell structure constituted by a polymer of a polyester resin having at least one polar group and a compound having at least one polymerizable unsaturated double bond, and a receiving layer composed of this dispersion.
However, it has been difficult to achieve satisfactory detachability from the thermal transfer-receiving sheet by merely forming a core/shell structure constituted by resins having glass transition temperatures differing within a particular range as described in Japanese Unexamined Patent Application Publication No. 10-337967. Similarly, it has been difficult to achieve satisfactory detachability by a resin having a core/shell or domain structure constituted by a urethane resin that is not particularly limited and a polymerizable unsaturated compound (vinyl-containing polymer) that is not particularly limited as in Japanese Unexamined Patent Application Publication Nos. 10-309874, 10-337967, and 10-337969. In addition, these resin compositions could not offer good image storage property such as lightfastness and image bleeding resistance. The situation has been more or less the same with the dispersion having a core/shell structure constituted by a polymer of a polar group-containing polyester resin and a compound having a polymerizable unsaturated double bond as in Japanese Unexamined Patent Application Publication No. 06-24152.